I may already have posted this, but if I did it's long enough ago that a reminder won't hurt.
A horse that "balks" is refusing to go in the direction his rider or handler wants. A balking horse might stop and plant his hooves, or might go in other directions - spin to go the other way, backwards, sideways, even up (rearing).
Balking is an expression of a strong desire not to go in the intended direction and is most often caused by not wanting to move away from the herd. It can also be caused by the horse being uncomfortable with the ground - often, horses will balk if asked to go into water if they cannot see the bottom, some horses won't walk through mud puddles, etc.
Basically, a horse that balks is saying "I don't want to."